1. Field of the Invention
The present invention relates to a gas turbine, and in particular, to a gas turbine starting control system capable of increasing starting power while suppressing a rise in inlet gas temperature of the turbine during starting of the gas turbine.
2. Description of the Related Art
FIG. 3 is a block diagram schematically showing a general gas turbine. In FIG. 3, reference numeral 1 indicates a compressor for compressing intake gas (air) 2, reference numeral 5 indicates a turbine coaxially provided with the compressor 1, and reference numeral 4 indicates a combustor. High-pressure air compressed by the compressor 1 is first delivered through a compressed-air path 13 to the combustor 4 where fuel is injected into the high-pressure air for combustion. As a result, combustion gas is generated. This combustion gas is then delivered to the turbine 5 through a combustion gas path 12 to perform expansion work by acting on moving blades of the turbine.
The gas turbine with the above mentioned arrangement generally exhausts some of the intake gas 2 (air) sucked into the compressor 1 to the outside through an extraction pipe 3 during starting of the gas turbine to prevent a rotating stall of the compressor 1. Accordingly, since the gas supplied to the turbine 5 through the combustor 4 has a smaller flow rate during starting of the gas turbine, there is a need for auxiliary power to start rotation of the rotational shaft of the compressor 1 and turbine 5. Consequently, a starting motor 6 is connected to the rotational shaft of the compressor 1 and turbine 5, and the power of the starting motor 6 is supplied thereto as an auxiliary power for starting the gas turbine.
FIG. 2 illustrates changes in an inlet gas temperature of the turbine with respect to the number of revolutions during starting of the above mentioned gas turbine provided with a starting motor. In the conventional gas turbine, as indicated by line B in FIG. 2, the inlet gas temperature of the turbine reaches it's maximum point at about 50% of rated speed and may exceed a predetermined allowable value. Moreover, line A in FIG. 2 indicates changes in the inlet gas temperature of the turbine with respect to the number of revolutions during starting of a gas turbine in accordance with the present invention, to be described later.
Nevertheless, in conventional gas turbines sufficient cooling air is generally not supplied to the moving blades and the like during starting of the gas turbine. Accordingly, the inlet gas temperature of the turbine must be kept at the above-mentioned allowable value (see FIG. 2) or lower. Consequently, in the conventional gas turbines described above, the starting motor 6 must have a large capacity in order to prevent an excess turbine load at the above-described 50% of rated speed. Accordingly, there are problems in that the space required to install such a large, high capacity starting motor is increased which increases the cost of the system as well.